Comparative Phytotoxicity of Metallic Elements on Duckweed Lemna gibba L. Using Growth- and Chlorophyll Fluorescence Induction-Based Endpoints

In this study, we exposed a commonly used duckweed species-<i>Lemna gibba</i> L.-to twelve environmentally relevant metals and metalloids under laboratory conditions. The phytotoxic effects were evaluated in a multi-well-plate-based experimental setup by means of the chlorophyll fluorescence imaging method. This technique allowed the simultaneous measuring of the growth and photosynthetic parameters in the same samples. The inhibition of relative growth rates (based on frond number and area) and photochemical efficiency (F_v/F_o and Y(II)) were both calculated from the obtained chlorophyll fluorescence images. In the applied test system, growth-inhibition-based phytotoxicity endpoints proved to be more sensitive than chlorophyll-fluorescence-based ones. Frond area growth inhibition was the most responsive parameter with a median EC₅₀ of 1.75 mg L^-1, while F_v/F_o, the more responsive chlorophyll-fluorescence-based endpoint, resulted in a 5.34 mg L^-1 median EC₅₀ for the tested metals. Ag (EC₅₀ 0.005-1.27 mg L^-1), Hg (EC₅₀ 0.24-4.87 mg L^-1) and Cu (EC₅₀ 0.37-1.86 mg L^-1) were the most toxic elements among the tested ones, while As(V) (EC₅₀ 47.15-132.18 mg L^-1), Cr(III) (EC₅₀ 6.22-19.92 mg L^-1), Se(VI) (EC₅₀ 1.73-10.39 mg L^-1) and Zn (EC₅₀ 3.88-350.56 mg L^-1) were the least toxic ones. The results highlighted that multi-well-plate-based duckweed phytotoxicity assays may reduce space, time and sample volume requirements compared to the standard duckweed growth inhibition tests. These benefits, however, come with lowered test sensitivity. Our multi-well-plate-based test setup resulted in considerably higher median EC₅₀ (3.21 mg L^-1) for frond-number-based growth inhibition than the 0.683 mg L^-1 median EC₅₀ derived from corresponding data from the literature with standardized <i>Lemna</i>-tests. Under strong acute phytotoxicity, frond parts with impaired photochemical functionality may become undetectable by chlorophyll fluorometers. Consequently, the plant parts that are still detectable display a virtually higher average photosynthetic performance, leading to an underestimation of phytotoxicity. Nevertheless, multi-well-plate-based duckweed phytotoxicity assays, combined with chlorophyll fluorescence imaging, offer definite advantages in the rapid screening of large sample series or multiple species/clones. As chlorophyll fluorescence images provide information both on the photochemical performance of the test plants and their morphology, a joint analysis of the two endpoint groups is recommended in multi-well-plate-based duckweed phytotoxicity assays to maximize the information gained from the tests.